INHIBITORY EFFECT OF PROGESTERONE ON OCCUPIED ESTROGEN-RECEPTORS OF ANTERIOR-PITUITARY AND UTERUS IN ADULT-RATS

The acute effects of a single progesterone injection on estradiol receptor (E2R) binding in the anterior pituitary and uterus were examined in the mature ovariectomized rat. Adult ovariectomized female rats receiving estradiol were injected with various doses of progesterone followed 1 h later by a final injection of 2 μg of estradiol. All animals were sacrificed 1 h after estradiol and the E2R binding in the nuclei and cytosol was determined. In the anterior pituitary, progesterone decreased total nuclear E2R in a biphasic inhibitory pattern, with maximal effects at 0.8 and 4 mg/kg body wt doses of progesterone and a smaller decrease with the 2 mg/kg body wt dose. This progesterone‐induced decrease of nuclear E2R was due to a decrease in the occupied form of nuclear E2R. The unoccupied nuclear E2R and cytosol E2R binding were not altered by progesterone administration. In the uterus, progesterone caused a dose‐dependent decrease in total nuclear E2R binding. This effect also occurred in the occupied form of E2R. The uterine unoccupied nuclear E2R and cytosol E2R were not altered by progesterone administration. These studies emphasize differences in the tissue specificity of progesterone action on occupied nuclear E2R which are presumably responsible for the mediation of estradiol action, and provide a biochemical basis for our previous observations of multiphasic effect of progesterone on gonadotropin secretion. Copyright © 1990, Wiley Blackwell. All rights reserved